In order to show the relevance of the model and to find estimates for some of the model parameters, the model has been calibrated by comparing simulation results to historical data from 1900 to 1990. As previously stated, some of the historical data are uncertain. However, our objective with the model is not the exact reproduction of past trends; rather, we will concentrate on the long-term trends. This chapter will not describe the data collection and calibration efforts in detail but, instead, will be devoted to the most relevant results.
Figure 29.5(a) shows both historical and simulated consumption rates for the 1900-2100 period. The model reproduces past consumption fairly well on the basis of historical regional GDP and population data (Klein-Goldewijk and Battjes 1997) and historical consumption data from the US Bureau of Mines (USGS 1999; USBM various years), Metallgesellschaft (various years) and International Institute for Iron and Steel (IIIS 1996) and Schmitz (1979). A closer look reveals some discrepancies for both iron/steel and MedAlloy. Further improvement is only possible by also introducing region-specific IU curves (in addition to region-specific income and population trends). Figure 29.5(b) shows that simulated secondary production rates as a fraction of total production also follow the historical estimates fairly well.
In the model, the empirically derived relationships between ore grade and cumulative production result in steadily declining ore grades (see Figure 29.5(d)). We also know empirically that the ore grade for several metals has decreased considerably. At the start of the 20th century, for instance, iron ore was generally mined in the USA at grades more than 60 per cent, while nowadays grades of 20 per cent are common. In South America and Africa, in contrast, high grades are still found. For copper ore, the average ore grade in the USA was about 3 per cent around 1900 while the current grade is about 0.9 per cent (USBM 1993). In other regions grades are sometimes higher and the spread is substantial, but evidence of declining grades also exists here. It should be noted that ore grade decline is due, not only to depletion, but also to a transition to cheaper open-pit mining. Figure 29.5(d) shows that for MedAlloy the model indicates a trend from a 7 per cent global average ore grade in 1900 to a 2.8 per cent grade in 1990. For iron/steel the trend is from 64 per cent to 55 per cent. Obviously, this seriously impacts mining waste which is even aggravated by the trend to open-pit mining with its higher amounts of overburden.
Finally, the model reproduces both overall energy use of the iron and steel sector (IEA 1999a, 1999b) and energy intensity very well (Figure 29.5(e); Worell et al. 1997; Hendricks et al. 1998). Historical energy intensity for various countries between 1980 and 1991 was between 20 and 50 gigaJoules per metric ton (GJ/MT) of crude steel, while rates of efficiency improvement varied between 0.0 per cent and 1.8 per cent per year. The model shows a global energy intensity of 20-30gJ in the same period (including recycling) and an annual improvement of 0.6 per cent.
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